I tow a 3000# travel trailer with my Rav4 Sport V6. I have towed about 10,000 miles and have averaged 10 to 12 mpg at 60 mph. I get about 27 to 29 mpg at 70 mph without the trailer. I think I should be getting about 15 to 18 mpg. What do others get pulling a 3000# trailer?

It's not primarily the weight of the trailer that counts but the shape. A rounded fiberglass one, such as Scamp or Castia will take less gas than a more upright one of "conventional" design.
We got 15 mpg on a 1,650 mile trip in Alaska towing at speeds that rarely hit 60 mph. Another member reported the same at 57 mph with a similar trailer.

So I'd say your mileage is normal. What's misleading is the V6 tows with such apparent ease you don't realize how much power, and therefore gasoline, you're actually using.

I have a 2010 Heartland Edge trailer which is a ultra lite weight - pretty aerodynamic but not an Airstream. The difference between my 10 to 12 and 15 is still 25 to 50%. I would be happy with 15 mpg. I just do not understand. I think I should get 15 to 18 mpg. I live in Colorado but my mileage number come from trips to Baltimore and Florida.

Aren't we supposed to tow in 4th? I ask because with my GMC Suburban they specifically say not to use overdrive when towing anything large.

This is what my owner's manual says on page 144:

When the lever is in the “D” position, the
automatic transmission system will select
the most suitable gear for running conditions
such as normal cruising, hill climbing,
hard towing, etc.
Always use the “D” position for better fuel
economy and quieter driving. If the engine
coolant temperature is low, the transmission
will not shift into the overdrive gear
even in the “D” position.

But then on the next page:

When towing a trailer, in order to maintain
engine braking efficiency, do not
use the “D” position.

Yeah, that's a big trailer for a RAV4, IMO. Also hard to believe you could keep it to 60 mph running Baltimore - FL on I-95 with cars flying by at 75+. Just a few mph can cost the same in mpg. Once again it's elusive because of how easy the V6 tows.

All you're trying to do with gear selection is prevent continual hunting. In Alaska I used D on downhills, 4 on the level & slight uphill and 3 for steeper grades.

I pull a 3000 lb (actual weight loaded for a long trip) Escape 17B, a fiberglass trailer. Over the last 33,000 miles my average has been 15MPG towing. I avoid interstates when ever possible, prefer secondary roads towing at 55 - 57MPH in 4th gear.